Up to this time, an epoxy or polyimide resin has been used as a laminate material for a multi-layer printed wiring board over a long period of time. Recently, however, a multilayer printed wiring board excellent in electrical characteristics has been demanded, because a high speed processing operation of a large scale computer requires to increase signal transmission speed. Particularly, a laminate material of a low dielectric constant is required to reduce the delay time of signal transmission and the thickness of a circuit thereby to obtain a high signal transmission speed and a high density multilayer printed wiring board. A laminate material having a lower dielectric constant based on polybutadiene has been proposed as a placement for the epoxy and polyimide resins as disclosed in Japanese Patent Publication No. 58-21926(1983).
The above-mentioned polybutadiene resin exhibits a dielectric constant lower than that of the polyimide resin and therefore is suitable for the multilayer printed wiring board requiring a high speed signal transmission. However, it is problematic in that the polybutadiene exhibits a tackiness in prepregs, a low mechanical strength and an insufficient heat resistance. Further, the serious disadvantage of the polybutadiene is that it is a flammable resin.
For suppressing the flammability of the polybutadiene a low molecular weight flame retardant of a crosslinking type is added to the polybutadiene as disclosed in Japanese Patent Publication No. 59-19023(1984) partly corresponding to U.S. Pat. No. 4,268,433. However, this low molecular weight crosslinking type flame retardant has problems in that it generally affects the electrical characteristics of the resultant material adversely and that it lowers the thermal decomposition initiating temperature which is a criterion of heat resistance.
The use of a high-molecular weight flame retardant of a crosslinking type such as alkenyl ethers and alkenoyl esters of poly(p-hydroxybromostyrene) is proposed as disclosed in Japanese Patent Application Laid-Open No. 61-243844(1986) corresponding to U.S. Ser. No. 854,507 filed on Apr. 22, 1986 now U.S. Pat. No. 4,784,917. These retardants exhibit a relatively low dielectric constant and are excellent in heat resistance, so that the combination of such flame retardant with the polybutadiene prepolymer provides a laminate material suitable for the production of a multi-layer printed wiring board of a low dielectric constant. However, since all of the side chains of a poly(p-hydroxybromostyrene) derivative contains a reactive unsaturated group, such as an alkenyl or alkenoyl group, crosslinking reaction proceeds nonuniformly with respect to crosslinking point distance during crosslinking by heating so that stress concentration is caused during curing, and the resultant cured product tends to be caused cracking by heat shock or heat cycle, in that, during crosslinking reaction by heating the density of the reactive groups in the poly(p-hydroxybromostyrene) derivative prepolymer locally increases near crosslinking points due to polymer effect to cause crosslinking reaction concentration. Once the crosslinking proceeds to some extent, the mobility of the polymer lowers to suppress the crosslinking reaction, thus the distances between crosslinking points become nonuniform, in other words, the crosslink densities of the resultant cured product are rendered uneven. Such that the residual stress during curing reaction concentrates locally and is likely to cause the resultant cured product cracking by thermal impact such as heat shock or heat cycle.